Method of preparing lightweight mirror module

ABSTRACT

What are disclosed are method and apparatus improvements in a heliostat for reflecting solar energy onto a collector, the heliostat having a main support structure with pivoting and tilting motors and controls and mirror modules for reflecting the solar energy. The improvement is characterized by one of a combination, or curved, or lightweight mirror module in which the curved mirror focuses the energy more precisely, attenuates differential expansion due to temperature change, yet is simple and economical to build and is light enough in weight to enable building larger modules for the heliostats, as well as building larger heliostats. The specific improvement is characterized by a curved mirror formed over a plurality of templates with longitudinal support beams holding it into the predetermined curvature with transversely extending structural beams and attachment bolt for attaching it to the heliostat. Also disclosed are specific preferred methods steps and structural components.

FIELD OF THE INVENTION

This invention relates to heliostates. More particularly, it relates toimproved mirror modules for the heliostats for reflecting the solarenergy onto a remote collector.

DESCRIPTION OF THE PRIOR ART

With the increased cost and scarcity of fossil fuels and other energysources, much work is being done to try to use solar energy. Inemploying solar energy, a plurality of heliostats reflect the solarenergy onto a remote collector.

Typical of such systems are those described in U.S. Patents. Forexample, U.S. Pat. No. 3,905,352 lists some fourteen earlier patentsranging from U.S. Pat. Nos. 260,657 through 3,469,837; and describes asystem for collecting and transferring usable solar heat by reflectingthe sun from heliostats on an elevated platform into a central receivingstation.

U.S. Pat. No. 3,892,433, inventor Floyd A. Blake, describes a directsolar hydroelectric integrated system and concentrating heliostats forsuch a solar system. U.S. Pat. No. 3,924,604 describes a solar energyconversion system in which pivotally mounted pads reflect energy onto aelevated tower collector.

Initiallly the heliostats were large structural elements with high costper unit area. As larger total area of reflecting surface became needed,much research was put into trying to reduce the cost per unit area. Thisresulted in using material such as plastic foam adhered onto a steelbacking with a mirror front. While this did reduce cost significantly,it introduced a thermal error in which, because of uncontrolleddifferential expansion or contraction of the materials as thetemperature changed, a bowing of the mirror module resulted. This madedifficult keeping the mirror module focused onto the collector so as tomost efficiently use the sun's energy by the collector.

All of the prior art attempts to solve this problem have resulted inintolerably increasing the cost of the mirror modules above about $2.40per square foot. In a co-pending application by Alfred Jerome Anderson,entitled "Structural Heliostat", Ser. No. 06/138,207, filed Apr. 7,1980, there was disclosed an improved heliostat that provided one way ofcuring the thermal instability through the use of front and backportions on the mirror module of the same materials so the expansion wasthe same. This improved version had the ability to withstand weathersuch as beating from hail, rain, wind and the like and enabled cantingthe mirror modules and provided an improved module from the standpointof economy. This improved module still had a relatively heavy weight permirror module, however and did not provide the following groups offeatures deemed desirable.

For example, it is desirable that the mirror modules have the followingfirst group of features:

1. The mirror module should be lightweight, for example, about half theweight of the prior art modules.

2. The mirror module should resist thermal distortion because ofdifferential expansion or contraction.

3. The mirror module should be simple and economically constructed fromeconomical, readily available materials.

4. The mirror module should enable building larger modules and largerheliostats for a given main structural support and drive system.

In addition to this first group of features, it would be advantageous ifthe mirror modules were curved for focusing of the solar energy onto atarget to simultaneously minimize the scattering of the solar raysbecause of differential expansion and contraction as the temperaturechanged.

It is particularly advantageous to provide the combination of a curvedmirror and lightweight mirror module, simultaneously alleviating theproblem of differential expansion, and providing simple and economicalconstruction of readily available materials, and enabling building andusing larger modules and hence larger heliostats.

Thus it can be seen the prior art has not been totally successful insolving the problem delineated hereinbefore and also providing thefeatures delineated hereinbefore.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide a method ofmaking a lightweight mirror module that achieves a plurality of thefeatures not heretofore provided while simultaneously solving theproblems of the prior art in providing all of the advantages of theprior art.

It is a specific object of this invention to provide an improvement in amethod that provides a lightweight mirror module that has about half theweight of the prior art modules, that solves the problem withdifferential thermal expansion and contraction and provides all of theadvantages provided by the prior art while alleviating the problems ofthe prior art.

These and other objects will become apparent from the descriptive matterhereinafter, particularly when taken in conjunction with the appendeddrawings.

In accordance with this invention there is provided an improved,lightweight mirror module for a heliostat for reflecting solar energyonto a collector and including a main support structure; means forpivoting and tilting the heliostat so as to keep the solar energyfocused onto the collector; and a plurality of mirror modules forreflecting the solar energy onto the collector, the improvementcomprising having the lightweight mirror module that is economicallyconstructed from readily available, economical materials and consistingessentially of: a mirror support structure connected with the back ofthe mirror and consisting essentially of:

i. a plurality of longitudinally extending beams having sufficientconnection surface and strength for supporting the mirror cantileveredfrom the main support structure of the heliostat; and

ii. a plurality of transversely extending beams connected to thelongitudinally extending beams carrying a mirror;

and attachment means connecting the mirror support structure to the mainsupport structure.

In accordance with another embodiment of this invention there isprovided a method of preparing a lightweight mirror module for aheliostat comprising the steps of laying a mirror face down on a stablesupport bed,

a. connecting a plurality of longitudinally extending beams with thebackside of the mirror such that a strip of support is provided at least1/2 inch wise at least each 6 inches; and connecting a plurality oftransversely extending beams to the plurality of respectivelongitudinally extending beams; and

b. connecting the respective attachment means to the transverselyextending beams for connection with the main support structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevational view, partly schematic, showing oneembodiment of this invention.

FIG. 2 is a schematic side elevational view emphasizing the curvature ofthe respective mirrors of the mirror module of FIG. 1.

FIG. 3 shows a typical prior art reflection of solar energy onto atarget.

FIG. 4 represents a reflection of the solar energy onto the target inaccordance with the embodiment of FIG. 2.

FIG. 5 is a plan view of a plurality of templates emplaced on asupporting bed in accordance with one embodiment of this invention.

FIG. 6 is a side elevational view of the templates of FIG. 5 emplaced ona support bed.

FIG. 7 shows the templates inverted in accordance with a further stepafter FIG. 6.

FIG. 8 shows a still further step from the embodiment of FIG. 7 in whicha mirror and lightweight mirror support structure have been connected.

FIG. 9 is a partial cross sectional view showing one of thelongitudinally extending beams connected to a metallic sheet on thebackside of the mirror.

FIG. 10 is a partial cross sectional view of another embodiment of theinvention in which the longitudinally extending beam is connected with aglass substrate on the back side of the mirror.

FIG. 11 is an isometric view of the backside of a mirror module of thisinvention without the attachment means.

FIG. 12 is a partial side elevational view showing an attachment meansfor attaching to the transversely extending beams of FIG. 11.

FIG. 13 is a schematic view of a heliostat showing the canting sequence.

DESCRIPTION OF PREFERRED EMBODIMENTS

As indicated hereinbefore, the usual system for employing solar energyincludes a collector 11, FIGS. 1 and 2 for receiving and using theradiant energy from the sun. The solar system also includes a pluralityof reflectors, or heliostats, for reflecting the radiant energy of thesun onto the collector 11. Ordinarily, in the prior art, the collectorwas supported on a tower to facilitate receiving the radiant energy froma plurality of heliostats spaced about the collector. For example, wherea plurality of three or more rows of the respective heliostats wereemployed, the tower was at least 100 feet high, or higher. Usually itwas 200 feet high.

In the prior art, the collector included a steam generator that producedsteam by heating water for use in a Rankine cycle engine; or included anarray of photovoltaic cells to produce electricity directly. Other typecollectors can be employed and it is immaterial to this invention as tothe nature of the collector. For example, where water was converted tosteam, the steam was passed through turbines rotating generatorsgenerating electricity. If desired, the collector may absorb the radiantenergy to convert it to heat for heating oil or other high boilingliquid that will be passed in heat exchange relationship with the wateror the like.

The nature of the towers is immaterial to this invention.

If desired, respective sensors for each heliostat can be employed todirect reflected beams onto the target to insure that the sunlight isreflected onto the collector by the heliostat. On the other hand, othersystems such as computer controlled heliostats are known. These types ofheliostat controls are well known and need not be described herein. Itis sufficient to note that the heliostat is pivoted and tilted tomaintain the solar rays reflected onto the target collector.

As noted hereinbefore, the heliostat and remainder of the solar systemwere frequently located in terrain experiencing wide diurnal andseasonal temperature variations. These temperature variations induceduncontrolled differential thermal contraction or expansion between themirror and supporting substrate between the lowest temperature and thehighest temperature and resulted in thermal stesses and curvatureeffects which degraded the reflected solar energy image at the target.The type and degree of degradation was a complex function of thetemperatures at assembly and maximum and minimum temperatures in situ.

Referring to FIGS. 1 and 2, there is illustrated a heliostat 13 inaccordance with an embodiment of this invention. The heliostat includesa main support structure 15 and means 17 for pivoting and tilting theheliostat containing a plurality of mirror modules 19.

The main support structure 13 includes a vertical support such as a postembedded in a foundation 21, like concrete. The post is ordinarily ofsteel pipe or the like. As will be apparent, any support structureadequate to withstand the loads imposed will be satisfactory.

The means 17 comprises the usual combination of motors, gears andpinions for rotating the heliostat with respect to the vertical axis ofthe post 19 and for rotating the horizontally extending arms 23 and,hence, tilting the heliostat, including the mirror modules 19. Theheliostat may be fastened in a vertical or horizontal position for beingstored overnight, during windstorms and the like. Suitable fasteningmeans such as latches and the like can be employed to take the strainoff the means 17 for pivoting and tilting the heliostat. The means 17pivots and tilts the heliostat to keep the solar energy focused on thecollector. This focusing may be done, as indicated, by either sensors orby computer controls. The system employed is relatively immaterial tothis invention. As indicated hereinbefore, this invention is concernedwith a combination mirror module that is thermally stabilized againstcurvature induced by temperature change, has lightweight and that has acurved mirror, as well as the other features delineated hereinbefore.For example, as can be seen in FIG. 3, the prior art type reflectionresulted in diffuse patterns 25, 27. These diffuse patterns, or images,resulted from differential temperature expansion of the mirror modulesresulting in curvature and separated the respective images. In contrast,as can be seen in FIG. 4, the image 29 is coherent and concentrated whenreflected in accordance with the mirror module of this invention.

The method of embodiment of this invention may be understood byreferring to FIGS. 5-8. Specifically, the method of preparing a mirrormodule for a heliostat or the like comprises a plurality of steps asfollows. First a plurality of templates 31, 31a, 31b, and 31c, forexample, are prepared. The templets 31-31c are sized and placed so as tosupport a mirror in the predetermined curvature along a plurality ofpredetermined lines. As can be seen in FIG. 7, the templates arepreferably inverted and placed on a support bed 33. The predeterminedcurvature to be induced into the mirror is that of a sphere having aradius equal to twice the distance from the mirror module to thecollector. For all practical purposes, the radius of curvature of thesphere will be twice the distance from the main support structure of theheliostat in its array around the collector, to the collector. Initiallythe templates 31-31c were formed of cellulosic material such asfiberboard and the like. It was found, however, that there was a problemof degree of moisture adsorption such that the relative thickness,hardness and the like would change. Consequently, the templates wereformed of plastic such as sheets of polyethylene and the like.

The templates were emplaced on a firm support bed 33, as impliedhereinbefore. In fact, preferably, the templates were inverted asillustrated in FIG. 7, similarly as implied hereinbefore, before themirror was placed face down on the templates. Thereafter, the mirror waslaid face down on the templates such that the mirror had thepredetermined curvature for reflecting solar rays closely toward atarget spot on the collector when installed on the heliostat. Theinstallation of the mirror 35 can be seen in FIG. 8.

Subsequently, a mirror support structure 37 is connected with the backside of the mirror so as to maintain the predetermined curvature whenthe mirror is moved into position for being attached to the heliostat13. Any mirror support structure that will maintain the predeterminedcurvature and that will obviate the problems of curving because ofdifferential expansion or contraction may be employed herein. In apreferred embodiment, the support structure comprises a plurality oflongitudinally extending beams 39, FIG. 11, connected with the mirror,and a plurality of transversely extending beams 41 connected with thelongitudinally extending beams carrying the curved mirror. While thelongitudinally extending beams and the transversely extending beams maybe formed of any material that will bear the weight of the mirrorcantilevered from the main support structure of the heliostat, it ispreferred that they be metallic in order to be easily worked, treated toresist corrosive effects of the weather, and be readily connectable withthe backside of the mirror 35. In a particularly preferred embodimentthe longitudinally extending beams 39 comprise beams of the so-calledhat shape cross section shape. As illustrated in FIGS. 9 and 10, thebeams have a crown portion 43 with a small horizontally extending brimportion 45. The brim portions 45 are affixed, as by adhering, either tothe backside of the mirror 35 directly, or may employ a metallic backingsheet 47 as illustrated in FIG. 9.

When a metallic backing sheet 47 is employed, a layer of grease such asa silicone grease 75 may be employed between the mirror 35 and thebacking sheet 47. The silicone grease serves to protect the mirror backsurface from moisture or other environmental damage; it serves as anadhesive; and it permits differential thermal expansion to occur betweenthe glass mirror and metallic backing sheet which prevents thermalstresses or thermal curvatures from occurring. A metallic edge molding76 which is applied with a soft adhesive 77 such as butyl rubber,silicone rubber, or polysulfide elastomer serves to seal the edge fromrain and other environmental effect, and to securely affix the mirrorglass and metallic backing sheet together. The soft edge adhesive 77provides a good mechanical bond yet still permits any differentialexpansion or contraction to freely occur. The resultant configurationillustrated in FIG. 9 is, therefore, a very thermally stableconfiguration and temperature changes will not induce any curvatures ordistortions which would degrade the reflected beam quality.

When a metallic backing sheet is not employed, as is illustrated in FIG.10, the brim portions 45 may be affixed directly to the back of themirror 35. However, such a configuration would provide poor protectionof the backside mirror surface from environmental damage. To overcomethis potential problem special, highly reflective mirrors having glasssubstrates 47a, FIG. 10 can be employed. With these types mirrors, thefront glass is very thin with a resultant increased reflectivity forreflecting more of the solar energy that is incident thereon onto thecollector. The glass substrate 47a is adhered to the mirror by any oneof the conventionally employed laminating methods and such mirrors withaffixed glass back sheets are commercially obtainable. While suchlaminated mirrors are more expensive than ordinary mirrors, the addedcost is usually more than compensated by the increased reflectivitywhich results in more energy being delivered to the collector. Thelaminated glass is also stronger and more durable. Since the glasssubstrate serves to seal-in the mirror silvering layer, no additionalprotection such as the use of silicone grease and a metallic substrateis required. The brim portion 45 of the longitudinally extending beams39 are directly bonded to the laminated mirror. Any of the suitablebonding materials can be employed. One of the preferred types of bondingmaterials is a room-temperature-vulcanizing silicone rubber such asRTV-548-556A base and RTV-548-557B curing agent manufactured by GeneralElectric Company, Silicone Products Department, Waterford, N.Y. Anothersuitable type is the polyacrylic adhesive and catalyst such as Versilok204, available from Hughson Chemicals, Erie, Pa. Other suitableadhesives are well known in this art and need not be detailed at lengthherein. These include the polymethacrylic polymers and catalysts; theresins such as the epoxy resins, urethane resins, cyanoacrylate resins,methylacrylate resins, vinyl ester resins and the acrylate resins.Suitable catalysts, also referred to as accelerators or initiators,include the N,N-dimethyl-p-toluidine, N,N-dimethyl aniline, or, for theepoxy resins, cobalt naphthenate; and methyl ethyl ketone peroxide. Asis recognized these type polymers set up and adhere when they aresubjected to the accelerator or catalyst and are readily available fromseveral sources, as described in the aforementioned patent applicationSer. No. 138,207. Details of that application are incorporated herein byreference for details that are omitted herefrom. While the configurationof FIG. 10 employing longitudinally extending beams adhesively bondeddirectly to a mirror, either plain or laminated, results in a very lowcost, low weight reflective unit, this configuration does have thedisadvantage that temperature changes will cause curvature changes.However, in many large-scale applications the collector 11 is oflikewise a large size and the curvature change with temperature can betolerated. One of the advantages of this invention is that the curvedmirror, in and of itself, tends to alleviate problems with thedifferential temperature expansion and contraction; i.e., theundesirable convex curvature which causes beam divergence can be avoidedby judiciously pre-curving the mirror with sufficient concavity.

Hence, the concepts embodied herein are: a lightweight, low costconfiguration illustrated by FIG. 9 which offers the added advantage ofbeing thermally stable such that no curvature change occurs withtemperature; and an alternative configuration illustrated by FIG. 10which is even lighter in weight, and lower in cost but has thedisadvantage of curvature changes with temperature changes.

In any event, the longitudinally extending beams 39 are connected withthe transversely extending beams 41 by any of the conventional means.Such conventional means include thermal bonding such as welding,brazing, silver soldering; adhering, as by the adhesives delineatedhereinbefore, or bradding or riveting through flanged portions or thelike. As illustrated, the transversely extending beams 41 are cut out tomatch the longitudinally extending beams and have flanges that increasethe surface bonding between the two sets of beams. If desired, ofcourse, the transversely extending and longitudinally extending beamsmay be bradded together, welded together, or the like.

One of the advantages of this lightweight structure serving as themirror support structure is that the longitudinally extending beams, andeven the transversely extending beams, can be suitable cambered (curved)during a roll forming operation or fabricated straight and then notchedalong their length to be affixed to the backside of the mirror so thatthe mirror maintains the induced curvature even when connected with aheliostat main structural support by the attachment means 49, FIG. 12.

In the attachment means of FIG. 12, the bolt 49 has its head end 51affixed, as by welding, brazing or the like, to the transverselyextending beam 41 and extends to penetrate through an aperture 53 thatis larger than the diameter of the bolt 49. This allows for differencesin expansion and contraction of the main structural support and of themirror module 17. To accommodate this, a pair of spherical nuts 55 andspherical washers 57 are employed to allow a slight pivotal movement tothe bolt 49 through the aperture 53. If desired, of course, the base maybe pressed into the mirror module and a jam nut screwed downwardly on anentirely threaded bolt. Then the spherical nut and spherical washersthat are illustrated allow accommodating pivotal motion on the maintruss 59. As is recognized the main truss 59 is a conventionallyemployed truss hanging as a part of the main structural support of theheliostat.

In operation, the mirror module is formed as delineated hereinbefore.Once the attachment means are connected to the transversely extendingbeams 41, the mirror modules are ready to be affixed to the remainder ofthe main structure 15 of the heliostat, FIG. 13. As illustrated in FIG.13, the mirror modules are canted on a horizontally positioned set oftrusses to facilitate assembly, rather than being put in a verticalposition and focused onto the collector. In terms of focusing, theoutside edges of the respective mirror modules are attached slightlycloser to the collector in order to focus the rays toward the collector.For example, if the delineated heliostat structure 15, FIG. 13, is thenumber one heliostat, a Starret level equivalent to the number one rowwill be employed to effect focusing of the mirrors with respect to thehorizontal component of the focusing, or horizontal plane. As can beseen therein all of these Starret levels 61 for the horizontal focusingare labeled by the numeral 1 to show that it is the first row at apredetermined radial distance from the collector that has this focus.Expressed otherwise, the mount at which the outside edge is raised isapproximately the tangent, or sine, of the angle of inclination, orfocusing, with respect to the normal axis of the heliostat multiplied bythe distance from the center line of the heliostat to the particularattachment point. As can be seen in FIG. 11, it is preferred to formfour of the attachment points 64 with this invention to increasestructural torsional rigidity of the mirror modules. In the prior art ithas been conventional to employ only three attachment means. As will beunderstood, a heliostat is ordinarily about 24 feet square. Accordingly,it is frequently advantageous to focus the mirror modules from top tobottom, also. Accordingly, the Starret levels 63 will be for effectingthe focusing of the top and bottom mirror modules, labeled 2 in FIG. 13.By similar reasoning, the penultimate mirror modules toward,respectively, the top and bottom will be canted using Starret levels 65,labeled 3. By similar reasoning, the four centermost mirror modules inthe illustrated heliostat of FIG. 13 use Starret levels 67 designated bythe numeral 4. In accordance with conventional practice, therefore, therespective canting of the mirror modules is effected with the respectiveStarret levels such that all of the mirror modules in a given row ofheliostats the same radial distance from the collector have the samefocusing with respect to the horizontal axis when the heliostat is in avertical position. Similarly, the respective mirror modules havefocusing with respect to the top and bottom rows of mirror modules, nowtwo each, have the same canting. Similarly the next to the top and nextto the bottom rows have the same canting with respect to the verticalaxis when the heliostat is in a vertical position; and the fourcentermost mirror modules have the same canting with respect to thevertical axis. As will become apparent, the four centermost modules arecanted less than, for example, the four top and bottom mirror modules inorder to obtain focusing on the collector at a predetermined radialdistance. This focusing coupled with the curved mirror effects theexcellent results delineated in FIG. 4 in accordance with thisinvention. As implicit from the foregoing, the respective spherical nutsand washers can be moved inwardly along the bolt to effect the desiredfocusing with the Starret levels.

As soon as the mirror modules are connected with the structure of theheliostat, the respective heliostats are integrated into the centralcontrol system to start reflecting the sun onto the collector whendesired. For example, the heliostats may be stored at night and bycomputer or the like brought to reflect the sunlight onto the collectorin the morning as soon as the sun rises. As will be appreciated, ifthere is some error in reflection, the canting can be adjusted by simplyscrewing the spherical nuts along the bolt of the attachment means.

The following example illustrates the preferred embodiments of thisinvention.

EXAMPLE

In this invention, a conventional mirror was adhered to a metallicsubstrate 47 by silicone grease. As is recognized and as described inthe aforementioned Ser. No. 138,207, the silicone greases are employedto have water repellancy and have adequate strength to support themirror, yet have sufficient shear tolerance to permit differentialexpansion of the mirror and substrate when the temperature changeswithout inducing stresses or curvature effects. The silicone grease alsois adapted to hold the mirror securely and prevent fluttering of themirror with respect to the substrate. Of course, conventionallyavailable mirrors with attached substrates can be employed. Thetemplates 31-31c were cut and laid on the bed 33. The mirror was laidface down over them and the mirror support structure 37 attached asnoted hereinbefore; namely through adhesion of the transversely andlongitudinally extending beams to, respectively, the beams and the backof the mirror. The attachment means were affixed to the transverselyextending beams 41 and were attached to the main heliostat structure 15as delineated. Thereafter, the heliostat was connected into the maincontrol system and a plurality of these heliostats were directed on thetarget. Displays such as illustrated in FIG. 4 with coherent images wereobtained. In this embodiment, the longitudinally extending beamscomprised galvannealed steel hat beams have brims of one-half inchlength with six inch crown portions in between. The longitudinallyextending members were placed so that there was only six inches betweenthe edges of the beams. This guaranteed that at least each six inchesthere was a support seam at least one-half inch wide. This was found tobe adequate to give the necessary properties to mirrors which were 0.093inches thick when 0.020 inch thick steel longitudinal beams wereemployed. Similarly, the transversely extending beams were formed of0.020 inch thick galvanized steel that had flanges cut out to receivethe hat beams and had a U-shaped top portion that received theattachment means. With this structure the long hat beams weighed only 40pounds (10 pounds for each beam) and the cross members weighed onlythree pounds for two cross beams. The mirrors weighed 56 pounds so atotal of 99 pounds was all that a 4.0 foot wide by 12.0 foot long mirrormodule weighed. This corresponds to a unit weight of only 2.06 lb/ft2which is approximately one-half the unit weight of conventional mirrormodules.

With this same lightweight structure, highly reflective mirrors formedwith only 0.025 inch front mirror with 0.068 glass back sheet wereemployed. The glass encased the silvering on the mirror. Highreflectivity was obtained with this mirror.

Both sets of mirrors were tested for the equivalent of high wind loads.In horizontal position they were required to withstand 12 pounds persquare foot. They were tested and broke at 38 pounds per square foot. Inthe vertical position, they are required to withstand 37 pounds persquare foot. Again they were tested and broke at 38 pounds per squarefoot. To test against hail impact, the mirrors were pelted by one inchdiameter hailstones having a velocity in the range of 75-100 feet persecond. The mirrors were not damaged. It appears by having thestructural support at least each six inches, the mirrors resisted the 75feet per second velocity of one inch ice balls required for theheliostats. In addition, the pre-curved mirror modules attenuated theproblem of differential expansion and obtained satisfactory focusing ata range from 32° F. to 120° F.

This example indicated that there was a slight loss in torsionalstiffness requiring four tie down points instead of the three tie downpoints. Conventional Starret leveling could be employed.

This invention indicated that the mirror modules could be made inlengths of up to 24 feet instead of the conventional lengths of 12 feet.Moreover, because of the lightweight the sizes of the heliostats can beincreased and still be driven by the conventional drive units nowemployed.

From the foregoing, it can be seen that this invention accomplishes theobjects delineated hereinbefore. Specifically, this invention has all ofthe features delineated hereinbefore as desirable and not heretoforeprovided.

What is claimed is:
 1. A method of preparing a mirror module for aheliostat or the like for reflecting solar energy onto a collector,which includes laying a mirror face down on a firm supporting bed andattaching a supporting structure to the backside of the mirror, theimprovement comprising the steps of:a. adhesively connecting a pluralityof longitudinally extending beams to the backside of the mirror so as toprovide a region of support longitudinally of said mirror at least eachsix inches of transverse distance of said mirror; said longitudinallyextending beams being connected and having sufficient strength tosupport said mirror cantilevered from a heliostat main supportstructure; said longitudinally extending beams having a cross-sectionalshape of the so-called hat type and having brim areas of said beamsadhered to the back side of said mirror so as to provide saidlongitudinal region of support on each edge of said longitudinallyextending beam; and having a dimension such that there is no more thansix inches space between any region of support longitudinally of saidmirror in order that said mirror have the requisite hail resistance; b.connecting a plurality of transversely extending beams to saidlongitudinally extending beams carrying said mirror; said plurality oftransversely extending beams being adapted to share substantially evenlythe weight of said mirror and said longitudinally extending beams andminimize torsional effects in said mirror module; and c. affixing tosaid transversely extending beams a plurality of attachment means forattaching to the heliostat structure; said attachment means beingadapted to minimize flexure and hold a predetermined shape of saidmirror module.
 2. The method of claim 1 wherein said attachment meansare adapted to accomodate slight pivotal connection at the point ofconnection at one of their ends and to provide Starrett leveling andeach includes an all-thread bolt that is affixed at one end and has twonuts with two spherical washers intermediate the two nuts to accomplishthe Starrett type leveling for focusing of the solar energy reflected bysaid mirror.
 3. The method of claim 1 wherein said mirror backside has aglass substrate for protection of the mirroring and said beams connectdirectly to said glass substrate on the backside of said mirror.
 4. Themethod of claim 3 wherein said supporting structure is adhered to saidsubstrate.